"The average temperature of the ocean surface water is about 17 °C (62.6 F)."

17 °C

"So even though surface waters can be a comfortable 20 degrees Celsius (good for swimming in!), the majority of our ocean water has a temperature between 0-3 degrees Celsius (32-37.5 degrees Fahrenheit)."

0-3 °C

The ocean is an enormous body of salt water that covers about 70% of the Earth's
surface. The ocean has an area of 361 million square kilometers, it has a volume
over 1,340 million cubic kilometers and it has an average depth of 3,711 meters.
The World Ocean consists of five other oceans, which are the Arctic Ocean, Atlantic
Ocean, Indian Ocean, Pacific Ocean, and Antarctic or Southern Ocean. Less than
2% of the Earth's surface water is freshwater the rest, is just saltwater.

Only the first few tens or so meters of water is able to absorb solar radiation
causing different temperature at different surface. This is called the photic
zone. The temperature here usually varies with latitude. In the Persian Gulf
which is located near the equator, water can be as warm as 36 degrees Celsius
(98.6 degrees Fahrenheit). Water temperatures can be low as -2 degrees Celsius
(28.4 degrees Fahrenheit) at the Arctic and Antarctic Seas. The average temperature
of the entire ocean surfaces usually ranges from 15 to 17 degrees Celsius (59
to 62.9 degrees Fahrenheit).

There is a barrier between the surface water and deeper layers of the ocean
that are not mixed. The barrier begins around 100 meters and can extend another
few hundred meters downwards. There is a thermo cline meaning there is a quick
change in temperature when entering the deeper surface of water. A CTD (Conductivity,
Temperature and Depth) instrument (usually placed in the water from a ship or
a platform) measures temperature in the deep ocean. Recordings have shown that
the average temperature of water ranges from 0-3 degrees Celsius (32–37.5
degrees Fahrenheit).

Back in 1899 it was estimated that 92% of the ocean floor had a temperature
of lower than 40 degrees Fahrenheit. However, as time progressed global warming
(phenomenon where the average annual temperature of the Earth increases) is
changing the ocean's average temperature. In places such as the tropical seas,
surface temperatures have increased as much as 0.5 degrees Celsius (1 degree
Fahrenheit) between the 1990s and the mid-1960s. As a result, there were serve
hurricane increases, such as Hurricane Katrina in 2005. The Arctic and Antarctic
Sea ice are also melting due to warmer temperatures of water. This leads to
an increase in water levels of the ocean, further increasing global heating.
Alpine glaciers are also melting, which results in an increase in the freshwater
runoff to local human populations. It also decreases the water in salinity.

Hence, there really is no real average temperature of the ocean due to different
surface waters and global warming.

Global Mean Monthly Surface Temperature Estimates for the Base Period 1901 to 2000